The central hypothesis of this proposal is: that the different effects on activity, growth, fattening, and reproduction elicited by the time-of-day of meal feeding are produced to a great degree by alterations in the phase relationships of endogenous hormone rhythms; that the change sin physiological postures of activity (migration), growth, fattening, and reproduction noted in vertebrates with seasonal cycles are similarly produced; and that, therefore, meal feeding (specifically the timing of meal feeding) may be an excellent tool to manipulate and examine the phase relationshis of the circulation rhythms of prolactin, cortisol, thyroxine, triiodothyronine, and insulin as they relate to activity, growth, fattening and reproduction. A secondary hypothesis of this proposal is that the phase shifting effects of meal feeding are mediated through diet induced time-of-day changes in the availability of one or more of the precursors to serotonin, dopamine, norepinephrine or acetylcholine within the central nervous system. This study will examine the interrelationship of the daily activity rhythm; the daily rhythms of cortisol, prolactin, thyroid hormones and insulin; lipid and nonlipid weight gain; and reproduction in fishes in response to meal feeding. Fish will be subjected to a feeding regime wherein they receive a single daily meal at a specific time of day. Variables to be examined for fish on each feeding regime include: growth (weight gain and increase in standard length), fattening (chloroform/methanol extracted lipids), activity (remote-monitored), reproductive state (gonadal-somatic index and histology), and circulating hormone titers (radioimmunoassay). In addition, several studies using pharmacological and dietary manipulations of neurotransmitters and their precursors will examine the role of specific neurotransmitters in the phase-shifting ability of meal feeding. The results of these studies will add data important to an understanding of the circadian mechanisms involved in the response of vertebrates to a meal-fed diet as well as an understanding of the circadian mechanisms involved in growth, fattening and reproduction.